Foliar feeding done right: the 6 rules that decide whether it works
The best foliar feed fizzles out if the spray mix is not right. Water, pH and concentration decide whether the nutrient gets into the leaf or just sits on it. Six rules sort that out – they apply to every foliar fertiliser.
Rule 1: Clean water
Use rainwater or demineralised water. Hard water brings carbonates with it, and carbonates bind nutrients before they reach the leaf. Water hardness should be 5 to 15 °dH (German degrees).
If you only have hard water, a reverse osmosis unit is the more thorough solution. It costs money; rainwater is the cheaper route. In my opinion, this is the most common mistake in foliar feeding: the fertiliser is good, the water is not.
Have your water hardness measured once before you mix the next batch.
Rules 2 and 3: pH and EC of the spray mix
Set the pH of the spray mix to 5 to 6.5. In this range the leaf takes up nutrients best.
The EC value shows how much salt is in the mix. With repeated applications, stay below 3.8 mS. Depending on crop and timing, 6 to 9 mS is also possible, but leaf burn cannot be ruled out then.
The reason lies in the leaf itself: leaf sap has an EC of about 9 to 12 mS. If your mix gets close to that, you leave the leaf no margin. So measure the EC before you go out, not afterwards.
Rules 4 and 5: Coverage and moisture
The aim is to cover around 80% of the leaf area. Then the nutrient spreads evenly over the leaf instead of sitting in just a few spots.
Then time matters. Chelated nutrients are taken up by the leaf within a few hours. For that, the leaf has to stay moist for 2 to 3 hours. If the mix dries on before then, the nutrients stay on the surface.
So spray early in the morning or in the evening, when evaporation is low – especially in high summer. In rain, heavy dew or with a leaf temperature above 25 °C, leave it.
Rule 6: Microbiology and carbon
Add microbiology and a carbon source to the mix. Both are in compost tea. The plant then takes up the nutrients better, and root growth is stimulated.
Without compost tea, molasses goes into the spray mix as the carbon source – and only there. In the compost tea itself it would be a mistake: the sugar is immediately available, and the oxygen collapses between hour 8 and 12. I described the difference in the post on biostimulants from your own farm.
Compost tea only keeps for about 4 hours. So only make up the mix when you can go out. It belongs on the leaf when soil and plant are active. In a cold spring, microbiology works better via the soil – that is what compost extract is for.
What compost tea cannot do: compensate for hard water or the wrong pH. Rules 1 to 3 remain the basis.
Why the chelate belongs on the leaf
A chelate is a metal ion bound to a larger organic molecule. The bond is stable and yet soluble. The metal does not precipitate and stays available to the plant.
With foliar spraying, part of the mix always lands on the soil. Take a zinc fertiliser with EDTA, a synthetic chelator. In the soil, EDTA releases the zinc and binds iron instead. The zinc then combines with phosphorus and is no longer available to the plant.
Then there is the leaf: synthetic chelators are phytotoxic to leaves, organic ones are not. That is why organically bound micronutrients are the better choice for foliar application. Ours are built that way, for example in Carbo-Eco-Mg and in the boron foliar fertiliser.
The order in the tank
The order matters when mixing, too. This is how we add the components to the tank:
Whether two products are compatible, you check beforehand in a small batch. The order does not replace this test.
The next step
Start with the water. Have the hardness measured and check the pH of the mix before your next pass.
Whether the nutrient arrives in the leaf is shown by the leaf sap analysis: it measures what has arrived up top, not what you sprayed. What that looks like for magnesium is in the post on magnesium deficiency, for boron in the post on boron deficiency. Questions about the mix for your crop: get in touch for advice.
